Grain 15N of crops applied with organic and chemical fertilizers in a four-year rotation

Abstract

Variations in crop grain and soil N isotope composition (δ15N) in relation to liquid hog manure (δ15N of total N was +5.1‰), solid cattle manure (+7.9‰) and chemical fertilizer (+0.7‰ for urea and −1.9‰ for ammonium phosphate) applications, and control (no fertilizer application) were examined through a 4-year crop rotation under field conditions. Canola (Brassica napus), hull-less barley (Hordeum vulgare), wheat (Triticum aestivum), and canola were grown sequentially from 2000 (year 1) to 2003 (year 4). From year 2, hog manure or chemical fertilizers, but not cattle manure, treatments increased grain N concentrations over the control. Grain δ15N (+0.3 to +2.5‰) of crops applied with chemical fertilizers was lower than those in the other treatments, reflecting the effects of the N source with a lower δ15N, while the manure treatments tended to increase grain δ15N. The higher grain δ15N of crops applied with hog manure (+5.6 to +8.4‰) than those applied with cattle manure (+2.2 to +4.1‰) reflected the higher N availability of liquid hog manure (up to 70% as NH 4 + ) than solid cattle manure (99% organic N) and higher potentials for ammonia volatilization loss in hog manure rather than differences in manure δ15N signatures. Soil total- and extractable-N concentrations and δ15N tended to vary with the application of N sources with different N isotope composition and availability. Our study expanded the application of the δ15N technique for detecting N source (organic vs chemical) effects on N isotopic composition to field conditions and across a 4-year rotation, and revealed that N availability played a greater role than the δ15N signature of N sources in determining crop δ15N under the studied conditions.